L. Forró szerk.: Miscellanea Zoologica Hungarica 7. 1992 (Budapest, 1992)

Nesemann, H.: Species composition and zoogeography of the invertebrate fauna at the lower reaches of the Lajta River

6. 1991 (A); Gattendorf (Lajtakáta) 30. 6. 1991 (A); Deutsch-Hasslau 6. 1991 (A); Rohrau 14. 5.1991 (A); Gerhaus 9. 5.1991 (A); Bruck, in a secondary branch 16. 7.1991 (A). This species occurs in high abundance in the whole course of the Lajta. It is the only member of the family Gammaridae, which occurs in the slowly running secondary branches. In the mainstream of the Lajta, G. roeseli was found sympatrically with Gammarus fossarum Koch, 1835. It is of South-eastern European origin (Straskraba 1958). Gammarus fossarum Koch, 1835 Localities: Lajta at the confluence with the Mosoni-Duna 6. 9.1991 (H); Nickelsdorf (Miklóshalma) 23. 6. 1991 (A/H); Lajta-megyei-csatorna (Komitatskanal) near Nickelsdorf (Mikóshalma) 9. 1991 (A/H); Zurndorf (Zurány) 23. 6.1991 (A); Gattendorf (Lajtakáta) 30. 6.1991 (A); Deutsch-Hasslau 6. 1991 (A); Rohrau 14. 5.1991 (A); Gerhaus 9. 5.1991 (A). This rheophilic species mainly lives in the rhithron rivers and streams. It is also wide-spread in the larger running waters of the upper Danube basin, because of their rhithron character. G. fossarum is present in several of the lowland rivers of the Danube plain, which have a relatively strong current and gravel bottom (Zala, Lajta). Most slowly running rivers were not inhabited by this species (Rába, Rábca). This distribution pattern is similar to that within the Lajta floodplain. Here G. fossarum only settles in the main stream and does not enter the deadwater reaches (Karaman & Pinkster 1977). Dikerogammarus haemobaphes (Eichwald, 1841) Locality: Mosoni-Duna at Mosonmagyaróvár (Ungarisch Altenburg) 13. 9.1991 (H). Pontocaspian Dikerogammarus species are characteristic members of the Danubian fauna. Only D. haemobaphes is distributed widely in the upper Danube. It does not enter the tributaries of the Kisalföld. Along the Lajta, this species is restricted to the confluence with the Mosoni-Duna like Viviparus acerosus. Synureüa ambuláns (Fr. Müller, 1846) Locality: Rohrau, in a deadwater pond 14. 5.1991 (A). This amphipod is distributed in some of the tributaries of the Black and Baltic sea (Straskraba 1958). It inhabits the reed zones of lowland rivers and lakes as well as the mud and organic matter in small stagnant waters and springs (Dudich 1927). Several populations with rudimentary eyes are known to live near groundwater (Straskraba 1958). S. ambuláns was often the only gammarid species found in ponds, which are poor in oxygen (Forró 1991). The species reaches its western distribution limit in Lajta floodplain. It is absent from the upper Danubian river basin (Straskraba 1958, Vornatscher 1965). Decapods Family Astacidae During the investigation period, crayfish were not observed in the study area on the Lajta. Entz (1909, 1914) recorded the occurrence of Astacus leptodactylus Eschscholz, 1851 near Mosonmagyaróvár. This species has a pontocaspian distribution (Brodski 1969, Albrecht 1983, Roth & Kinzelbach 1986) with a western border in the Kisalföld. The abundant crayfish was recently found in the main stream of the Danube throughout the Alföld plain. It still exists in the Lajta basin, where it lives in deep lakes of gravel pitches (T. Ofenböck, pers. comm.).